In the field of horology, a timepiece movement of conventional architecture may also include a striking mechanism. This striking mechanism may be provided for generating at least one sound, via a gong, which is struck by a hammer at determined times in a striking watch to indicate a programmed alarm or minute repeaters. The gong is generally a metal wire of circular shape, which surrounds part of the watch movement inside a watch frame. The gong is fixed to a gong-carrier, which is integral with a bottom plate or with the watch case.
The striking mechanism may also be provided for generating music via a pin-barrel with several tongues, individually actuated by the pins of a cylinder or disc in a musical watch. The tongues of the pin-barrel are connected to the same heel, which may be fixed to the bottom plate or to the watch case, and the pins of the rotating cylinder or disc allow the free end of some tongues to be lifted. After the tongues have been bent via the action of the pins, the tongues are then released by the pins. The actuated tongues start to oscillate essentially at their first natural frequency.
The gong of the striking watch or the pin-barrel of the musical watch is arranged inside the watch case. Thus, the vibrations of the gong or the pin-barrel tongues are transmitted to the external parts of the watch. These external parts are, for example, the middle part, the bezel, the crystal and the back cover of the watch case. These large parts start to radiate sound into the air under the effect of the transmitted vibrations. When a sound is produced either by a gong struck by a hammer or by one or more vibrating pin-barrel tongues, these external parts are capable of radiating the produced sound into the air.
In a conventional striking or musical watch, acoustic efficiency, based on the complex vibro-acoustic transduction of the external parts, is low. In order to improve and increase the acoustic level perceived by the user of the striking or musical watch, the material, geometry and boundary conditions of the external parts must be taken into account. The configurations of these external parts are also dependent upon the aesthetic appearance of the watch and operating stresses, which may limit adaptation possibilities.
It is known in watchmaking technology to use an acoustic type membrane, which is dedicated to vibro-acoustic transduction, in a watch and particularly an electronic watch. To activate this type of membrane in an electronic watch, a piezoelectric element is, for example, placed on the membrane to cause it to vibrate, as mentioned in CH Patent No. 581 860. To prevent the acoustic radiation from the membrane from being lost in the watch, which must be sealed, a double back cover can be provided for the watch case, which must be open towards the exterior. In such case, the back cover of the watch case has one or more apertures for the transmission of sound from the vibrating membrane.
With this type of design for an electronic watch with an acoustic membrane, problems often arise relating to the sealing and corrosion of said membrane. The first natural vibration frequency of this membrane, which is the efficient radiation mode, must, in principle, be within the useful acoustic frequency band. However, the second natural frequency, which is an inefficient mode, must, if possible, be outside this audible band. The useful acoustic frequency band is generally between 1 kHz and 4 kHz. According to the boundary conditions and geometry set for the membrane, the physical properties, such as density and the Young's modulus, must allow the first and second natural frequencies to be adapted. If the membrane is made of steel, the first and second natural vibration frequencies do not fulfil the aforementioned conditions in an optimum manner. Moreover, rapid damping is observed, which is a drawback.
Generally, with the use of a conventional acoustic radiating membrane, a problem of frequency bandwidth exists. If the acoustic membrane has to be fitted to a music box, the frequencies to be radiated efficiently must typically range between 1 kHz and 4 kHz. In the case of a striking watch with minute repeaters, an alarm or even a quartz alarm, excellent results may be obtained by amplifying a single dominant frequency, tuned with the exciter.
In a standard striking watch, which is, for example, fitted with an acoustic membrane, the membrane is sandwiched between part of the middle case and the back cover of the watch. In the case of a luxury watch, the back cover may be made of a precious material, such as gold. A difference in electrochemical potential may occur on contact between the membrane, which is generally made of steel, and the gold back cover, especially in a humid environment. This is liable to contribute to the corrosion of said membrane where it is in contact with the gold back cover, which is another drawback. A corrosion resistant material must therefore be found which has no difference in potential with gold and low internal damping.